The present invention relates to silicon carbide (SiC) fibers, used as reinforcement in the production of composite materials.
The search for good mechanical properties of composite materials, in particular at high temperatures, has led to the use of ceramic materials for fibrous reinforcement, in place of carbon which shows a limited mechanical strength and insufficient resistance to oxidation in the event of a prolonged time spent in an oxidizing medium at high temperature.
Generally, it is well known that, in composite materials comprising a fibrous reinforcement, the characteristics of the fiber-matrix interface have a great influence on the mechanical properties of the material.
In the case of composite materials comprising a ceramic matrix, it has been shown that satisfactory behavior can be obtained, in particular with respect to impacts and to the propagation of cracks, and despite the ceramic nature of the matrix, by forming on the fibers an intermediate coating of low thickness, for example made of pyrolytic carbon deposited in the vapor phase or made of boron nitride, before infiltration of the ceramic material of the matrix. Such a process is described in French patent FR 2 567 874.
However, it is advantageous to improve the bonding interface between the deposited layer of pyrolytic carbon or of boron nitride and the silicon carbide fibers. It is also advantageous to be able to have silicon carbide fibers which have reinforced mechanical properties.
It is already known that the formation of a superficial layer of microporous carbon at the surface of silicon carbide fibers has advantages in terms of improving the mechanical properties at ambient temperature (U.S. Pat. No. 6,579,833, WO 2005/007566) and constitutes, in addition, a good bonding interface with the deposited layer of pyrolytic carbon (WO 2010/076475).
However, the process described in these documents uses a reactive thermal treatment based on a halogenated gas generating a layer of microporous carbon at the surface of the carbide. This process is therefore complicated to carry out from the point of view of both the process itself and the means for carrying it out, which must be resistant to halogenated compounds.
In addition, some silicon carbide fibers have at their surface a thin layer of silica which it is advisable to remove before any subsequent treatment.
A treatment for chemical removal of the silica present at the surface of silicon carbide fibers is known from patent application FR 2 640 258. However the treatment recommended, which consists of the use
either of hydrofluoric acid in solution in water,
or of a solution of nitric acid in water under hot conditions,
or else of a mixture of hydrofluoric acid, nitric acid and acetic acid in the precise proportions (HF 3 ml+HNO3 5 ml+CH3—CO2H 3 ml) under hot conditions, which corresponds to an HF/HNO3 molar ratio of 1.29,
does not allow total removal of the layer of silica present at the surface of the fibers: the layer is only reduced to a thickness of about 0.005 micron, which corresponds to the limit of detection.
It would therefore be advantageous to be able to have a process which allows both the total removal of the layer of silica present at the surface of the fibers and the formation of a superficial layer of microporous carbon.
The use of hydrofluoric acid or a mixture of hydrofluoric acid and nitric acid as a solution for washing SiC fibers during the production thereof is also known, from patent application EP 0 855 373. However, this washing step is not a chemical treatment step since it serves only to remove the residues of the unreacted initial products present at the surface of the fibers after the production thereof, such as particles of silicon dioxide (or silica). It does not therefore allow the formation of a layer of microporous carbon on these fibers. In addition, this document discloses only an aqueous solution containing 46% by weight of HF mixed with an equivalent content of a solution of HNO3 containing from 60% to 70% by weight of HNO3. Example 2 thus discloses a mixture of 250 ml of the aqueous solution of HF with 250 ml of the solution of HNO3. Thus, the acid solution of this example has a nitric acid content of between 4.76 and 5.54 mol/l and an HF content of approximately 11.5 mol/l, i.e. a hydrofluoric acid to nitric acid molar ratio of between 2.08 and 2.41. The acid concentrations described in this document are therefore so strong that it is impossible to control the reaction.